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<tr class="t-nv"><td colspan="5"> <a href="attributes.html" title="cpp/language/attributes"> Attributes</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion"> Implicit conversions</a> - <a href="explicit_cast.html" title="cpp/language/explicit cast"> Explicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> - <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
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<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="classes.html" title="cpp/language/classes"> Classes</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> General </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Overview</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> <code>class</code>/<code>struct</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"> <code>union</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="injected-class-name.html" title="cpp/language/injected-class-name"> Injected-class-name</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Members </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html" title="cpp/language/data members">Data members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">Static members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this">The <code>this</code> pointer</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">Nested classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">Member templates</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bitfield.html" title="cpp/language/bit field">Bit fields</a></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"><code>using</code>-declarations</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">Member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">Member access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor">Constructors and member initializer lists</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">Default member initializer</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> specifier</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">Converting constructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/copy assignment">Copy assignment operator</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move assignment">Move assignment operator</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">Destructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">Base and derived classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ebo.html" title="cpp/language/ebo">Empty base optimization (EBO)</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">Virtual member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">Pure virtual functions and abstract classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"><tt>override</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"><tt>final</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Introduces a name that is defined elsewhere into the declarative region where this using-declaration appears.
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>using</b></code> <code><b>typename</b></code><span class="t-mark">(optional)</span> <span class="t-spar">nested-name-specifier</span> <span class="t-spar">unqualified-id</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-until-cxx17">(until C++17)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>using</b></code> <span class="t-spar">declarator-list</span> <code><b>;</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">


<tr class="t-par">
<td>  <code><b>typename</b></code>
</td>
<td> -
</td>
<td>  the keyword <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> may be used as necessary to resolve <a href="dependent_name.html" title="cpp/language/dependent name">dependent names</a>, when the using-declaration introduces a member type from a base class into a class template
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">nested-name-specifier</span>
</td>
<td> -
</td>
<td>  a sequence of names and scope resolution operators <code>::</code>, ending with a scope resolution operator. A single <code>::</code> refers to the global namespace.
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">unqualified-id</span>
</td>
<td> -
</td>
<td>  an <a href="name.html" title="cpp/language/identifiers">id-expression</a>
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">declarator-list</span>
</td>
<td> -
</td>
<td>  comma-separated list of one or more declarators of the <code><b>typename</b></code><span class="t-mark">(optional)</span> <span class="t-spar">nested-name-specifier</span> <span class="t-spar">unqualified-id</span>. Some or all of the declarators may be followed by an ellipsis <span class="t-c"><span class="mw-geshi cpp source-cpp">...</span></span> to indicate <a href="parameter_pack.html" title="cpp/language/parameter pack">pack expansion</a>
</td></tr></table>
<h3><span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Using-declarations can be used to introduce namespace members into other namespaces and block scopes, or to introduce base class members into derived class definitions<span class="t-rev-inl t-since-cxx20"><span>, or to introduce <a href="enum_class.html" title="cpp/language/enum">enumerators</a> into namespaces, block, and class scopes</span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>A using-declaration with more than one using-declarator is equivalent to a corresponding sequence of using-declarations with one using-declarator.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h4><span class="mw-headline" id="In_namespace_and_block_scope">In namespace and block scope</span></h4>
<p>Using-declaration introduces a member of another namespace into current namespace or block scope.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
 
<span class="kw1">using</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    string str <span class="sy1">=</span> <span class="st0">"Example"</span><span class="sy4">;</span>
    <span class="kw1">using</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a><span class="sy4">;</span>
    cout <span class="sy1">&lt;&lt;</span> str<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>See <a href="namespace.html" title="cpp/language/namespace">namespace</a> for details.
</p>
<h4><span class="mw-headline" id="In_class_definition">In class definition</span></h4>
<p>Using-declaration introduces a member of a base class into the derived class definition, such as to expose a protected member of base as public member of derived. In this case, <span class="t-spar">nested-name-specifier</span> must name a base class of the one being defined. If the name is the name of an overloaded member function of the base class, all base class member functions with that name are introduced. If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">struct</span> B
<span class="br0">{</span>
    <span class="kw1">virtual</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"B::f<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span>        <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"B::g<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span>         <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"B::h<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">protected</span><span class="sy4">:</span>
    <span class="kw4">int</span> m<span class="sy4">;</span> <span class="co1">// B::m is protected</span>
    <span class="kw1">typedef</span> <span class="kw4">int</span> value_type<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D <span class="sy4">:</span> B
<span class="br0">{</span>
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">m</span><span class="sy4">;</span>          <span class="co1">// D::m is public</span>
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">value_type</span><span class="sy4">;</span> <span class="co1">// D::value_type is public</span>
 
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"D::f<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// D::f(int) overrides B::f(int)</span>
 
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">g</span><span class="sy4">;</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"D::g<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// both g(int) and g(char) are visible</span>
 
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">h</span><span class="sy4">;</span>
    <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"D::h<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// D::h(int) hides B::h(int)</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    D d<span class="sy4">;</span>
    B<span class="sy3">&amp;</span> b <span class="sy1">=</span> d<span class="sy4">;</span>
 
<span class="co1">//  b.m = 2;  // Error: B::m is protected</span>
    d.<span class="me1">m</span> <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>  <span class="co1">// protected B::m is accessible as public D::m</span>
 
    b.<span class="me1">f</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls derived f()</span>
    d.<span class="me1">f</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls derived f()</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"----------<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    d.<span class="me1">g</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls derived g(int)</span>
    d.<span class="me1">g</span><span class="br0">(</span><span class="st0">'a'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls base g(char), exposed via using B::g;</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"----------<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
    b.<span class="me1">h</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls base h()</span>
    d.<span class="me1">h</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>   <span class="co1">// calls derived h()</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">D::f
D::f
----------
D::g
B::g
----------
B::h
D::h</pre></div></div> 
</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<h4> <span class="mw-headline" id="Inheriting_constructors">Inheriting constructors</span></h4>
<p>If the <i>using-declaration</i> refers to a constructor of a direct base of the class being defined (e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">Base</span><span class="sy4">;</span></span></span>), all constructors of that base (ignoring member access) are made visible to overload resolution when initializing the derived class. 
</p><p>If overload resolution selects an inherited constructor, it is accessible if it would be accessible when used to construct an object of the corresponding base class: the accessibility of the using-declaration that introduced it is ignored.
</p><p>If overload resolution selects one of the inherited constructors when initializing an object of such derived class, then the <code>Base</code> subobject from which the constructor was inherited is initialized using the inherited constructor, and all other bases and members of <code>Derived</code> are initialized as if by the defaulted default constructor (default member initializers are used if provided, otherwise default initialization takes place). The entire initialization is treated as a single function call: initialization of the parameters of the inherited constructor is sequenced-before initialization of any base or member of the derived object.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B1 <span class="br0">{</span> B1<span class="br0">(</span><span class="kw4">int</span>, ...<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B2 <span class="br0">{</span> B2<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span>   <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D1 <span class="sy4">:</span> B1
<span class="br0">{</span>
    <span class="kw1">using</span> B1<span class="sy4">::</span><span class="me2">B1</span><span class="sy4">;</span> <span class="co1">// inherits B1(int, ...)</span>
    <span class="kw4">int</span> x<span class="sy4">;</span>
    <span class="kw4">int</span> y <span class="sy1">=</span> get<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> test<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    D1 d<span class="br0">(</span><span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: B1 is initialized by calling B1(2, 3, 4),</span>
                   <span class="co1">// then d.x is default-initialized (no initialization is performed),</span>
                   <span class="co1">// then d.y is initialized by calling get()</span>
 
    D1 e<span class="sy4">;</span>          <span class="co1">// Error: D1 has no default constructor</span>
<span class="br0">}</span>
 
<span class="kw1">struct</span> D2 <span class="sy4">:</span> B2
<span class="br0">{</span>
    <span class="kw1">using</span> B2<span class="sy4">::</span><span class="me2">B2</span><span class="sy4">;</span> <span class="co1">// inherits B2(double)</span>
    B1 b<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
D2 f<span class="br0">(</span><span class="nu16">1.0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: B1 has no default constructor</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> W <span class="br0">{</span> W<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> X <span class="sy4">:</span> <span class="kw1">virtual</span> W
<span class="br0">{</span>
    <span class="kw1">using</span> W<span class="sy4">::</span><span class="me2">W</span><span class="sy4">;</span> <span class="co1">// inherits W(int)</span>
    X<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Y <span class="sy4">:</span> X
<span class="br0">{</span>
    <span class="kw1">using</span> X<span class="sy4">::</span><span class="me2">X</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Z <span class="sy4">:</span> Y, <span class="kw1">virtual</span> W
<span class="br0">{</span>
    <span class="kw1">using</span> Y<span class="sy4">::</span><span class="me2">Y</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
Z z<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: initialization of Y does not invoke default constructor of X</span></pre></div></div>
<p>If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed, similar to multiply-inherited non-static member functions:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> A<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span> <span class="kw1">using</span> A<span class="sy4">::</span><span class="me2">A</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> C1 <span class="sy4">:</span> B <span class="br0">{</span> <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">B</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> C2 <span class="sy4">:</span> B <span class="br0">{</span> <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">B</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D1 <span class="sy4">:</span> C1, C2
<span class="br0">{</span>
    <span class="kw1">using</span> C1<span class="sy4">::</span><span class="me2">C1</span><span class="sy4">;</span>
    <span class="kw1">using</span> C2<span class="sy4">::</span><span class="me2">C2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
D1 d1<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// ill-formed: constructor inherited from different B base subobjects</span>
 
<span class="kw1">struct</span> V1 <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">B</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> V2 <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">B</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D2 <span class="sy4">:</span> V1, V2
<span class="br0">{</span>
    <span class="kw1">using</span> V1<span class="sy4">::</span><span class="me2">V1</span><span class="sy4">;</span>
    <span class="kw1">using</span> V2<span class="sy4">::</span><span class="me2">V2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
D2 d2<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: there is only one B subobject.</span>
          <span class="co1">// This initializes the virtual B base class,</span>
          <span class="co1">//   which initializes the A base class</span>
          <span class="co1">// then initializes the V1 and V2 base classes</span>
          <span class="co1">//   as if by a defaulted default constructor</span></pre></div></div>
<p>As with using-declarations for any other non-static member functions, if an inherited constructor matches the signature of one of the constructors of <code>Derived</code>, it is hidden from lookup by the version found in <code>Derived</code>. If one of the inherited constructors of <code>Base</code> happens to have the signature that matches a copy/move constructor of the <code>Derived</code>, it does not prevent implicit generation of <code>Derived</code> copy/move constructor (which then hides the inherited version, similar to <code>using operator=</code>).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B1 <span class="br0">{</span> B1<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B2 <span class="br0">{</span> B2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D2 <span class="sy4">:</span> B1, B2
<span class="br0">{</span>
    <span class="kw1">using</span> B1<span class="sy4">::</span><span class="me2">B1</span><span class="sy4">;</span>
    <span class="kw1">using</span> B2<span class="sy4">::</span><span class="me2">B2</span><span class="sy4">;</span>
 
    D2<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: D2::D2(int) hides both B1::B1(int) and B2::B2(int)</span>
<span class="br0">}</span><span class="sy4">;</span>
D2 d2<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>    <span class="co1">// calls D2::D2(int)</span></pre></div></div>
<p>Within a <a href="templates.html" title="cpp/language/templates">templated class</a>, if a using-declaration refers to a <a href="dependent_name.html" title="cpp/language/dependent name">dependent name</a>, it is considered to name a constructor if the <span class="t-spar">nested-name-specifier</span> has a terminal name that is the same as the <span class="t-spar">unqualified-id</span>. 
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="sy4">:</span> T
<span class="br0">{</span>
    <span class="kw1">using</span> T<span class="sy4">::</span><span class="me2">T</span><span class="sy4">;</span> <span class="co1">// OK, inherits constructors of T</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw1">struct</span> B <span class="sy4">:</span> T, A<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span>
<span class="br0">{</span>
    <span class="kw1">using</span> A<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">A</span><span class="sy4">;</span> <span class="co1">// OK, inherits constructors of A&lt;U&gt;</span>
    <span class="kw1">using</span> T<span class="sy4">::</span><span class="me2">A</span><span class="sy4">;</span>    <span class="co1">// does not inherit constructor of T</span>
                   <span class="co1">// even though T may be a specialization of A&lt;&gt;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<h4> <span class="mw-headline" id="Introducing_scoped_enumerators">Introducing scoped enumerators</span></h4>
<p>In addition to members of another namespace and members of base classes, using-declaration can also introduce enumerators of <a href="enum_class.html" title="cpp/language/enum">enumerations</a> into namespace, block, and class scopes.
</p><p>A using-declaration can also be used with unscoped enumerators.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw2">enum</span> <span class="kw1">class</span> button <span class="br0">{</span> up, down <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw1">using</span> button<span class="sy4">::</span><span class="me2">up</span><span class="sy4">;</span>
    button b <span class="sy1">=</span> up<span class="sy4">;</span> <span class="co1">// OK</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">using</span> button<span class="sy4">::</span><span class="me2">down</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> button non_up <span class="sy1">=</span> down<span class="sy4">;</span> <span class="co1">// OK</span>
 
<span class="kw4">constexpr</span> <span class="kw4">auto</span> get_button<span class="br0">(</span><span class="kw4">bool</span> is_up<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">using</span> button<span class="sy4">::</span><span class="me2">up</span>, button<span class="sy4">::</span><span class="me2">down</span><span class="sy4">;</span>
    <span class="kw1">return</span> is_up <span class="sy4">?</span> up <span class="sy4">:</span> down<span class="sy4">;</span> <span class="co1">// OK</span>
<span class="br0">}</span>
 
<span class="kw2">enum</span> unscoped <span class="br0">{</span> val <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">using</span> unscoped<span class="sy4">::</span><span class="me2">val</span><span class="sy4">;</span> <span class="co1">// OK, though needless</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Notes">Notes</span></h3>
<p>Only the name explicitly mentioned in the using-declaration is transferred into the declarative scope: in particular, enumerators are not transferred when the enumeration type name is using-declared.
</p><p>A using-declaration cannot refer to a namespace<span class="t-rev-inl t-until-cxx20"><span>, to a scoped enumerator</span> <span><span class="t-mark-rev t-until-cxx20">(until C++20)</span></span></span>, to a destructor of a base class or to a specialization of a member template for a user-defined conversion function.
</p><p>A using-declaration cannot name a member template specialization (<a href="templates.html#template-id" title="cpp/language/templates">template-id</a> is not permitted by the grammar):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D <span class="sy4">:</span> B
<span class="br0">{</span>
    <span class="kw1">using</span> B<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>      <span class="co1">// OK: names a template</span>
<span class="co1">//  using B::f&lt;int&gt;; // Error: names a template specialization</span>
 
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> f<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>A using-declaration also can't be used to introduce the name of a dependent member template as a <i>template-name</i> (the <code>template</code> disambiguator for <a href="dependent_name.html" title="cpp/language/dependent name">dependent names</a> is not permitted).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span>
<span class="kw1">struct</span> B
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> Y<span class="sy1">&gt;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> B<span class="sy1">&lt;</span>Y<span class="sy1">&gt;</span>
<span class="br0">{</span>
<span class="co1">//  using B&lt;Y&gt;::template f; // Error: disambiguator not allowed</span>
    <span class="kw1">using</span> B<span class="sy1">&lt;</span>Y<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>          <span class="co1">// compiles, but f is not a template-name</span>
 
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>
    <span class="br0">{</span>
<span class="co1">//      f&lt;int&gt;(0);          // Error: f is not known to be a template name,</span>
                            <span class="co1">// so &lt; does not start a template argument list</span>
        f<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>               <span class="co1">// OK</span>
    <span class="br0">}</span>   
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If a using-declaration brings the base class assignment operator into derived class, whose signature happens to match the derived class's copy-assignment or move-assignment operator, that operator is hidden by the implicitly-declared copy/move assignment operator of the derived class. <span class="t-rev-inl t-since-cxx11"><span>Same applies to a using-declaration that inherits a base class constructor that happens to match the derived class copy/move constructor</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>The semantics of inheriting constructors were retroactively changed by a <a href="using_declaration.html#Defect_reports">defect report against C++11</a>. Previously, an inheriting constructor declaration caused a set of synthesized constructor declarations to be injected into the derived class, which caused redundant argument copies/moves, had problematic interactions with some forms of SFINAE, and in some cases can be unimplementable on major ABIs. Older compilers may still implement the previous semantics.
</p>
<div style="margin-left:0px">
<table class="mw-collapsible mw-collapsed" style="background: transparent; text-align: left; border: 1px solid silver; margin: 0.2em auto auto; width:100%; clear: both; padding: 1px;">

<tr>
<th style="background: #F0F2F5; font-size:87%; padding:0.2em 0.3em; text-align:center;"> <span style="font-size:115%"> Old inheriting constructor semantics</span>
</th></tr>
<tr>
<td style="border: solid 1px silver; padding: 8px; background: white;">
<p>If the <i>using-declaration</i> refers to a constructor of a direct base of the class being defined (e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> Base<span class="sy4">::</span><span class="me2">Base</span><span class="sy4">;</span></span></span>), constructors of that base class are inherited, according to the following rules:
</p>
<div class="t-li1"><span class="t-li">1)</span> A set of <i>candidate inheriting constructors</i> is composed of </div>
<div class="t-li2"><span class="t-li">a)</span> All non-template constructors of the base class <span class="t-rev-inl t-since-cxx14"><span>(after omitting ellipsis parameters, if any)</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span></div>
<div class="t-li2"><span class="t-li">b)</span> For each constructor with default arguments or the ellipsis parameter, all constructor signatures that are formed by dropping the ellipsis and omitting default arguments from the ends of argument lists one by one</div>
<div class="t-li2"><span class="t-li">c)</span> All constructor templates of the base class <span class="t-rev-inl t-since-cxx14"><span>(after omitting ellipsis parameters, if any)</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span></div>
<div class="t-li2"><span class="t-li">d)</span> For each constructor template with default arguments or the ellipsis, all constructor signatures that are formed by dropping the ellipsis and omitting default arguments from the ends of argument lists one by one</div>
<div class="t-li1"><span class="t-li">2)</span> All candidate inherited constructors that aren't the default constructor or the copy/move constructor and whose signatures do not match user-defined constructors in the derived class, are implicitly declared in the derived class. The default parameters are not inherited:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B1
<span class="br0">{</span>
    B1<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D1 <span class="sy4">:</span> B1
<span class="br0">{</span>
    <span class="kw1">using</span> B1<span class="sy4">::</span><span class="me2">B1</span><span class="sy4">;</span>
 
    <span class="co1">// The set of candidate inherited constructors is </span>
    <span class="co1">// 1. B1(const B1&amp;)</span>
    <span class="co1">// 2. B1(B1&amp;&amp;)</span>
    <span class="co1">// 3. B1(int)</span>
 
    <span class="co1">// D1 has the following constructors:</span>
    <span class="co1">// 1. D1() = delete</span>
    <span class="co1">// 2. D1(const D1&amp;) </span>
    <span class="co1">// 3. D1(D1&amp;&amp;)</span>
    <span class="co1">// 4. D1(int) &lt;- inherited</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B2
<span class="br0">{</span>
    B2<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">13</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> D2 <span class="sy4">:</span> B2
<span class="br0">{</span>
    <span class="kw1">using</span> B2<span class="sy4">::</span><span class="me2">B2</span><span class="sy4">;</span>
 
    <span class="co1">// The set of candidate inherited constructors is</span>
    <span class="co1">// 1. B2(const B2&amp;)</span>
    <span class="co1">// 2. B2(B2&amp;&amp;)</span>
    <span class="co1">// 3. B2(int = 13, int = 42)</span>
    <span class="co1">// 4. B2(int = 13)</span>
    <span class="co1">// 5. B2()</span>
 
    <span class="co1">// D2 has the following constructors:</span>
    <span class="co1">// 1. D2()</span>
    <span class="co1">// 2. D2(const D2&amp;)</span>
    <span class="co1">// 3. D2(D2&amp;&amp;)</span>
    <span class="co1">// 4. D2(int, int) &lt;- inherited</span>
    <span class="co1">// 5. D2(int) &lt;- inherited</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<p>The inherited constructors are equivalent to user-defined constructors with an empty body and with a <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">member initializer list</a> consisting of a single <span class="t-spar">nested-name-specifier</span>, which forwards all of its arguments to the base class constructor.
</p><p>It has the same <a href="access.html" title="cpp/language/access">access</a> as the corresponding base constructor. It is <code>constexpr</code> if the user-defined constructor would have satisfied <code>constexpr</code> constructor requirements. It is deleted if the corresponding base constructor is deleted or if a defaulted default constructor would be deleted (except that the construction of the base whose constructor is being inherited doesn't count). An inheriting constructor cannot be explicitly instantiated or explicitly specialized.
</p><p>If two using-declarations inherit the constructor with the same signature (from two direct base classes), the program is ill-formed.
</p><p>An inheriting constructor template should not be <a href="function_template.html#Explicit_instantiation" title="cpp/language/function template">explicitly instantiated</a> or <a href="template_specialization.html" title="cpp/language/template specialization">explicitly specialized</a>.
</p>
</td></tr></table></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p><a href="parameter_pack.html" title="cpp/language/parameter pack">Pack expansions</a> in using-declarations make it possible to form a class that exposes overloaded members of variadic bases without recursion:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">&gt;</span>
<span class="kw1">struct</span> Overloader <span class="sy4">:</span> Ts...
<span class="br0">{</span>
    <span class="kw1">using</span> Ts<span class="sy4">::</span><span class="me2">operator</span><span class="br0">(</span><span class="br0">)</span>...<span class="sy4">;</span> <span class="co1">// exposes operator() from every base</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">T</span><span class="sy1">&gt;</span>
Overloader<span class="br0">(</span>T...<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">&gt;</span> Overloader<span class="sy1">&lt;</span>T...<span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// C++17 deduction guide, not needed in C++20</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">auto</span> o <span class="sy1">=</span> Overloader<span class="br0">{</span> <span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&amp;</span> a<span class="br0">)</span> <span class="br0">{</span><a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> a<span class="sy4">;</span><span class="br0">}</span>,
                         <span class="br0">[</span><span class="br0">]</span> <span class="br0">(</span><span class="kw4">float</span> f<span class="br0">)</span> <span class="br0">{</span><a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <a href="../io/manip/setprecision.html"><span class="kw1800">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> f<span class="sy4">;</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/258.html">CWG 258</a>
</td>
<td> C++98
</td>
<td> a non-const member function of a derived class can<br>override and/or hide a const member function of its base
</td>
<td> overriding and hiding also require<br>cv-qualifications to be the same
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1738.html">CWG 1738</a>
</td>
<td> C++11
</td>
<td> it was not clear whether it is permitted to explicitly instantiate or<br>explicitly specialize specializations of inheriting constructor templates
</td>
<td> prohibited
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.link/P0136R1">P0136R1</a>
</td>
<td> C++11
</td>
<td> inheriting constructor declaration injects<br>additional constructors in the derived class
</td>
<td> causes base class constructors<br>to be found by name lookup
</td></tr></table>

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